a. As a method for decomposing Michael addition products by-produced during production of acrylic acid or acrylic esters, it is common to employ a thermal decomposition method using no catalyst in the case of a process for producing acrylic acid (JP-A-11-12222), while in the case of a process for producing an acrylic ester, a method is known to carry out the decomposition by heating in the presence of a Lewis acid or a Lewis base (JP-A-49-55614, JP-B-7-68168, JP-A-9-110791, JP-A-9-124552, JP-A-10-45670). Further, as a decomposition reactive system for Michael addition products, it is common to employ a reaction distillation system wherein the desired decomposed reaction product is distilled by distillation while carrying out the decomposition reaction. Further, a method is also known wherein Michael addition products by-produced in a step for producing acrylic acid, and Michael addition products by-produced in a step for producing an acrylic ester, are put together, followed by thermal decomposition. There are a method for thermal decomposition by a reactive distillation system in the absence of any catalyst (JP-A-8-225486) and a method for decomposition by means of a highly concentrated acid catalyst (JP-A-9-183753).
In order to increase the recovery rate of acrylic acid, an acrylic ester or an alcohol useful as a product or as a raw material for a reaction, at the top of such a decomposition reaction column, it is necessary to increase the decomposition reaction temperature and to suppress the bottom discharge amount, whereby there has heretofore been a problem such that the bottom liquid tends to be a highly viscous liquid; as the decomposition temperature is high, an oligomer or polymer of acrylic acid or an acrylic ester being an easily polymerizable substance, is likely to form; and some of substances contained in the raw material for the reaction tend to precipitate, whereby a solid will deposit at the bottom of the decomposition reaction column, a polymer is formed due to a liquid contained in the deposit, and such a deposit will flow into a liquid discharge line at the time of an operational change thereby to cause sudden clogging of the liquid discharge line; and thus, there has been no appropriate method whereby the decomposition reaction column can be operated constantly for a long time. Especially when a solid has once deposited at the bottom of a decomposition reaction column, an easily polymerizable liquid occluded in the deposited solid tends to be extremely polymerizable since it can not flow, and the decomposition reaction temperature is relatively high, thus leading to a phenomenon where the amount of the deposit will be further increased by such polymerization. Thus, it has been desired to cope with this problem.
b. As an example to solve this problem, a method is conceivable wherein the diameter of a pipe to transfer the bottom liquid is reduced to transfer the liquid at a high flow rate, but it has been impossible to adopt such a method, since the pump for such a transfer is required to be of a high pressure type, such being economically disadvantageous as an industrial production method. Further, a method is also conceivable wherein in order to lower the viscosity of the bottom liquid, waste liquid from the production step may be added or water may be added afresh, but such will bring about a decrease of the liquid temperature, whereby clogging tends to be rather accelerated, or it tends to be required to add such water in a large amount. Accordingly, it has been practically impossible to adopt such a method.
c. On the other hand, as is well known, there is a vapor phase oxidation method of propylene as a reaction to form acrylic acid. For such a method of obtaining acrylic acid by oxidizing propylene, there are a two step oxidation process wherein oxidation to acrolein and a next step of oxidation to acrylic acid, are carried out in separate reactors, respectively, since the oxidation conditions are different, and a process wherein oxidation to acrylic acid is carried out directly by one step oxidation.
FIG. 9 shows an example of a flowchart for forming acrylic acid by two step oxidation, followed by a reaction with an alcohol to form an acrylic ester. Namely, propylene, steam and air are subjected to two step oxidation via the first and second reactors packed with e.g. a molybdenum-type catalyst to form an acrylic acid-containing gas. This acrylic acid-containing gas is contacted with water in a collection column to obtain an aqueous acrylic acid solution, which is extracted in an extraction column by adding a suitable extraction solvent, whereupon the extraction solvent is separated in a solvent separation column. Then, acetic acid is separated in an acetic acid separation column to obtain crude acrylic acid, and in a fractionating column, a byproduct is separated from this crude acrylic acid to obtain a purified product of acrylic acid. Further, this acrylic acid (purified product) is esterified in an esterification reaction column, and then, via an extraction column and a light component separation column, a crude acrylic ester is obtained. From this crude acrylic ester, a byproduct (high boiling product) is separated in a fractionating column to obtain a purified product of an acrylic ester.
Here, depending upon the type of the acrylic ester, there may be a case where the flow sheet will be as shown in FIG. 10. In such a case, the byproduct is obtained as bottoms in an acrylic acid separation column.
In the process for producing an acrylic ester in FIG. 10, acrylic acid, an alcohol, recovered acrylic acid and a recovered alcohol are respectively supplied to an esterification reactor. This esterification reactor is packed with a catalyst such as a strongly acidic ion exchange resin. An esterification reaction mixture comprising a formed ester, unreacted acrylic acid, an unreacted alcohol, formed water, etc., withdrawn from this reactor, will be supplied to an acrylic acid separation column.
From the bottom of this acrylic acid separation column, the bottom liquid containing unreacted acrylic acid is withdrawn and recycled to an esterification reactor. A part of this bottom liquid is supplied to a high boiling component separation column, whereby a high boiling component is separated from the bottom, and this is supplied to and decomposed in a high boiling component decomposition reactor (not shown). The decomposition product containing a valuable substance formed by the decomposition will be recycled to the process. A place in the process where the decomposition product is recycled, varies depending upon the process conditions. High boiling impurities such as polymers will be discharged from the high boiling decomposition reactor to the exterior of the system.
From the top of this acrylic acid separation column, an acrylic ester, an unreacted alcohol and formed water are distilled. A part of the distillate is recycled as a reflux liquid to the acrylic acid separation column, and the rest is supplied to an extraction column.
To this extraction column, water for extraction of an alcohol is supplied. Water containing an alcohol, flowing out of the bottom, will be supplied to an alcohol recovery column. The distilled alcohol will be recycled to the esterification reactor.
A crude acrylic ester discharged from the top of the extraction column will be supplied to a light boiling component separation column, and a light boiling material is withdrawn from the top and recycled within a process. A place within the process where it is recycled, varies depending upon the process conditions. The crude acrylic ester having the low boiling material removed, will be supplied to a purification column for an acrylic ester product, whereby a high purity acrylic ester will be obtained from the top. The bottom liquid contains a large amount of acrylic acid and therefore is recycled within the process. The place within the process where it will be recycled, varies depending upon the process conditions.
Further, in recent years, instead of a solvent extraction method wherein recovery of acrylic acid from the above aqueous acrylic acid solution is carried out by means of an extraction solvent, an azeotropic separation method is carried out wherein distillation is carried out by means of water and an azeotropic solvent, so that from the top of the azeotropic separation column, an azeotropic mixture comprising water and the azeotropic solvent, is distilled, and from the bottom, acrylic acid is recovered.
Further, also practically used is a method wherein acrylic acid is obtained by using propane instead of propylene and using a Mo—V—Te type composite oxide catalyst or a Mo—V—Sb type composite oxide catalyst. In the case of methacrylic acid and a methacrylic ester, isobutylene or t-butyl alcohol is employed instead of propylene, and a purified product of methacrylic acid and a purified product of a methacrylic ester are obtained via a similar oxidation process and the subsequent esterification process.
Further, as a method for forming a (meth)acrylic ester (an acrylic ester or a methacrylic ester), a method is practically employed wherein a (meth)acrylic ester of a lower alcohol and a higher alcohol are subjected to a transesterification reaction by using e.g. an acid as a catalyst, to produce a (meth)acrylic ester of the higher alcohol. The crude (meth)acrylic ester obtained by this transesterification exaction, is subjected to steps such as catalyst separation, concentration and fractionation to obtain a purified (meth)acrylic ester.
A useful byproduct such as a Michael addition product, is contained in the fraction separated by distillation and purification of the above-mentioned crude acrylic acid, the crude methacrylic acid, the crude acrylic ester or the crude methacrylic ester. Accordingly, this byproduct is decomposed to recover (meth)acrylic acid or its ester, or the raw material alcohol.
Heretofore, the methods as disclosed in the above a have been known as methods for decomposing a Michael addition product by-produced during production of acrylic acid or an acrylic ester. Thus, heretofore, it has been common to decompose a Michael addition product by-produced during production of an acrylic ester thereby to recover a valuable substance such as acrylic acid, an acrylic ester or an alcohol. As such a decomposition and recovery method, it has been common to employ a reactive distillation system wherein distillation is conducted while carrying out a decomposition reaction.
To carry out the reactive distillation system, a reactor provided at its upper portion with a distillation column, is employed. As such a distillation column, it is common to employ a plate column provided internally with various trays, or a packed column having various packing materials packed, in order to bring about fractionating effects. The plates may, for example, be bubble cap trays, uniflux trays, flexible trays, ballast trays, perforated trays (sieve trays), chimney trays, ripple trays, dual flow trays or baffle trays. The packing material may, for example, be a ring-type packing material such as Raschig rings, spiral rings or pall rigns, or a saddle type packing material such as Berl saddle or interlock saddle, or others such as Goodloe packing, Dixon ring, MacMahon packing, or a vertically flat plate type regulated packing material.
However, in both production processes for acrylic acid and an acrylic ester, the raw material to be supplied to the step of decomposing the byproduct, is a fraction obtained by concentrating a high boiling component formed in the reaction system or purification system. Further, acrylic acid and acrylic esters are very easily polymerizable materials, and consequently, the raw material for the decomposition reaction contains polymers formed. Further, the decomposition reaction is carried out at a high temperature, and therefore, there will be a polymer formed during the decomposition reaction. Accordingly, it is likely that a solid substance is already present in the raw material to be subjected to decomposition, and even when no solid substance is present in the raw material, it may precipitate anew, or there may be a solid substance to be formed during the distillation separation operation or in the decomposition step where a chemical reaction is simultaneously carried out. And, adhesion, deposition or accumulation of such a solid substance takes place on the trays or at void spaces of the packing material in the distillation column, whereby an increase of the differential pressure, deterioration of the gas/liquid contact state and further clogging, may, for example, occur. Consequently, there has been a problem that such tends to hinder to obtain a high recovery rate of a valuable substance or tends to hinder a constant continuous operation.
Accordingly, in both processes for producing acrylic acid and the ester, it is desired to solve the above problems and to develop a process for decomposing a Michael addition product, whereby a high recovery rate can constantly be obtained.
d. Further, in a method for recovering (meth)acrylic acid or a (meth)acrylic ester by carrying out the decomposition reaction of a Michael addition reaction product by-produced during the process for producing (meth)acrylic acid or a (meth)acrylic ester, if the decomposition reaction temperature is made high in order to obtain a high recovery rate for such (meth)acrylic acid, a (meth)acrylic ester or an alcohol, an oligomer or polymer of (meth)acrylic acid or a (meth)acrylic ester being an easily polymerizable substance, will be formed. To prevent such polymerization, it is suggested to add molecular oxygen in addition to a polymerization inhibitor such as hydroquinone, methoxyhydroquinone, phenothiazine or hydroxylamine, to the decomposition reactor (e.g. the above-mentioned JP-A-10-45670, paragraphs 0012 and 0019).
However, if such a method is employed, there may sometimes be a case where not only no adequate effect for preventing polymerization of (meth)acrylic acid or a (meth)acrylic ester in the decomposition product by oxygen is obtainable, but also polymerization may be accelerated, and thus there may be a case where the above decomposition reaction can not be constantly continued over a long time.
e. Further, an acrylic acid-containing gas obtained by vapor phase catalytic oxidation by molecular oxygen of propylene and/or acrolein, usually contains maleic acid, as one of byproducts, in an amount of from about 0.2 to 1.6 wt %, based on acrylic acid. Maleic acid is a dicarboxylic acid represented by HOCO—CH═CH—CO2H and is in an equilibrium state with a carboxylic anhydride having one molecule of water dehydrated in its molecule in its solution. Hereinafter, unless otherwise specified, maleic acid and maleic anhydride will be together represented by maleic acid. When an acrylic acid-containing gas is collected by a solvent in the form of an acrylic acid-containing solution, maleic acid will be collected at the same time. The boiling point of maleic acid is high as compared with acrylic acid, and in the purification step by distillation, maleic acid will be concentrated in the bottoms.
When two molecules of acrylic acid undergo Michael addition, an acrylic acid dimer will be formed. There is no means to prevent formation of such an acrylic acid dimer in the acrylic acid solution, and the formation speed increases as the temperature becomes high. Further, a higher oligomer such as an acrylic acid trimer will sequentially be formed by acrylic acid and an acrylic acid dimer. In the purification step for acrylic acid, an acrylic acid dimer (or oligomer) will be formed mostly in the distillation column where heating is carried out, particularly at the bottom portion where the temperature is high, and the retention time is long.
In order to improve the recovery rate of acrylic acid in the purification step, it is usual to recover acrylic acid from the formed acrylic acid oligomer.
As a recovery method from an acrylic acid oligomer, there may, for example, be a method wherein thermal decomposition is carried out under reduced pressure in the presence or absence of a catalyst, and acrylic acid is recovered as a distilled gas or a distilled liquid, as disclosed in JP-B-45-19281. In such a case, the distilled gas and the distilled liquid of acrylic acid contains a large amount of high boiling compounds other than acrylic acid to be recovered, such as maleic acid. In a case where the operation temperature is increased in order to increase the recovery rate of acrylic acid, the maleic acid concentration in the recovered acrylic acid will also be increased.
As a method to reduce such maleic acid, in a method as disclosed in JP-A-11-12222, a crude acrylic acid containing from 3 to 10 wt % of maleic acid and other acrylic acid oligomers, is introduced into an acrylic acid recovery column, and acrylic acid is distilled from the top, and the bottom liquid is thermally decomposed, and such a bottom liquid is recycled to the recovery column, whereby maleic acid can be reduced to a level of from 0 to 3 wt %.
In such a thermal decomposition recovery method of an acrylic acid oligomer, maleic acid as an impurity is disposed as bottoms of the thermal decomposition reaction apparatus or the distillation apparatus. At that time, if the amount of maleic acid contained in the recovered acrylic acid is large, the amount of maleic acid recycled in the system will increase, whereby instruments and the heat load in the purification step will increase. The simplest method to prevent this, is to reduce the thermal decomposition recovery amount of the acrylic acid oligomer, but the recovery rate for acrylic acid in the purification step will thereby be decreased, and the economical efficiency will be deteriorated.
In order to accomplish improvement of the recovery rate of acrylic acid and reduction of the recycling amount of maleic acid, there is a method of adding a distillation column as in the method disclosed in JP-A-11-1222. However, since acrylic acid is an easily polymerizable compound, it is common to carry out distillation under reduced pressure to prevent polymerization by lowering the operational temperature, but as the boiling point of maleic acid is higher than acrylic acid, even if the operation pressure is lowered, an increase of the operational temperature can not be avoided. This will not only facilitate clogging of the distillation apparatus by polymerization, but also tends to accelerate formation of an acrylic acid oligomer in the acrylic acid recovered by thermal decomposition. Further, in order to increase the vacuuming degree of the distillation installation, the diameter of the distillation column is increased, whereby the load during the construction and operation will also increase.
Further, concentrated maleic acid is discharged from the bottom. However, maleic acid is solid at room temperature and thus has problems such that the viscosity of the liquid tends to be high from the lower portion to the bottom of the distillation column, and deterioration in the separation ability due to fouling, or deposition of a polymer or clogging is likely to result.
Such problems result as maleic acid being an impurity is separated as a high boiling substance by distillation.
In order not to include a step of concentrating maleic acid by distillation and to improve the thermal decomposition recovery efficiency of acrylic acid, it is necessary to carry out, without imparting a large heat as distillation, either {circumflex over (1)} reducing the maleic acid concentration in the acrylic acid solution to be supplied to the thermal decomposition reaction apparatus, or {circumflex over (2)} reducing maleic acid in the acrylic acid solution recovered from the thermal decomposition reaction apparatus.
f. Further, heretofore, in an installation for producing acrylic acid or the like, it has been common to carry out a pressure measurement by installing a high pressure side detection portion of a liquid level meter in direct connection to the main body of the instrument. However, by a conventional method for installation of a liquid level meter, a polymerization inhibitor to be used for the preparation of an easily polymerizable compound or a formed polymer, is supplied to the high pressure side detection portion of the liquid level meter, and a solid substance is likely to be accumulated, whereby an error operation of the liquid level meter used to be observed.
Accordingly, it used to be difficult to carry out accurate measurement continuously by a liquid level meter, whereby it has been difficult to carry out a constant operation of the installation for a long period of time.